Solar cells are devices that convert the energy of light into electricity using the photovoltaic effect. Solar power is an attractive green energy source because it is sustainable and produces only non-polluting by-products. Accordingly, a great deal of research is currently being devoted to developing solar cells with enhanced efficiency while continuously lowering material and manufacturing costs.
When light hits a solar cell, a fraction of the incident light is reflected by the surface and the remainder is transmitted into the solar cell. The transmitted light/photons are absorbed by the solar cell, which is usually made of a semiconducting material, such as silicon. The absorbed photon energy excites electrons from the atoms of the semiconducting material, generating electron-hole pairs. These electron-hole pairs are then separated by p-n junctions and collected by conductive electrodes that are applied on the solar cell surface.
The most common solar cells are those based on silicon, more particularly, a p-n junction made from silicon by applying a dopant diffusion layer onto a silicon substrate, coupled with two electrical contact layers or electrodes. In a p-type semiconductor, doping atoms are added to the semiconductor in order to increase the number of free charge carriers (positive holes). The doping atoms remove weakly bound outer electrons from the semiconductor atoms. The purpose of p-type doping is to create an abundance of holes. In the case of silicon, a trivalent atom is substituted into the crystal lattice. One example of a p-type semiconductor is silicon with a boron or aluminum dopant. Solar cells can also be made from n-type semiconductors. In an n-type semiconductor, the doping atoms provide extra electrons to the host substrate, creating an excess of negative electron charge carriers. Such doping atoms usually have one more valence electron than one type of the host atoms. The most common example is atomic substitution in group IV solids (silicon, germanium, tin) which contain four valence electrons by group V elements (phosphorus, arsenic, antimony) which contain five loosely bound valence electrons. One example of an n-type semiconductor is silicon with a phosphorous dopant.
In order to minimize reflection of the sunlight by the solar cell, an antireflection coating (ARC), such as silicon nitride, silicon oxide, alumina oxide, or titanium oxide, is applied to the n-type or p-type diffusion layer to increase the amount of light absorbed into the solar cell. The ARC is typically non-conductive, and may also passivate the surface of the silicon substrate.
For silicon solar cell metallization processes, a rear contact is typically first applied to the silicon substrate. A typical process involves applying a back side silver paste or silver/aluminum paste to form soldering pads, followed by an aluminum paste applied to the entire back side of the substrate. Second, using an electroconductive paste composition, a metal contact may be screen printed onto the front side antireflection layer (after drying of the back side paste) to serve as a front electrode. This electrical contact layer on the front face or front of the cell, where light enters, is typically present in a grid pattern made of “finger lines” and “bus bars” rather than a complete layer because the metal grid materials are typically not transparent to light. The silicon substrate with printed front side and back side paste is then fired, e.g., at a temperature of approximately 700-975° C. After firing, the front side paste etches through the ARC layer, forms an electrical contact between the grid contacts and the semiconductor, and converts the metal paste to metal electrodes on the light receiving surface of the solar cell. The back side paste is typically fired at the same time as the front side paste, and forms an electrical contact with the backside of the silicon substrate. The resulting metallic electrodes allow electricity to flow to and from solar cells connected in a solar panel. See e.g., A. Luque and S. Hegedus, Eds., Handbook of Photovoltaic Science and Engineering, J. Wiley & Sons, 2nd Edition, 2011; P. Würfel, Physiks of Solar Cells, Wiley VCH, Verlag GmbH & Co. KGaA, Weinheim, 2nd Edition, 2009.
To assemble a solar module, multiple solar cells may be connected in series and/or in parallel and the ends of the electrodes of the first cell and the last cell are preferably connected to output wiring. The solar cells are typically encapsulated in a transparent thermal plastic resin, such as silicon rubber or ethylene vinyl acetate. A transparent sheet of glass is placed on the front surface of the encapsulating transparent thermal plastic resin. A back protecting material, for example, a sheet of polyethylene terephthalate coated with a film of polyvinyl fluoride having good mechanical properties and good weather resistance, is placed under the encapsulating thermal plastic resin. These layered materials may be heated in an appropriate vacuum furnace to remove air, and then integrated into one body by heating and pressing. Furthermore, since solar cells are typically left in the open air for a long time, it is desirable to cover the circumference of the solar cell with a frame material consisting of aluminum or the like.
A typical electroconductive paste contains metallic particles, a glass frit and an organic medium. These components are usually selected to take full advantage of the theoretical potential of the resulting solar cell. For example, it is desirable to maximize the contact between the metallic paste and silicon surface, and the metallic particles themselves, so that the charge carriers can flow through the interface and finger lines to the bus bars. The glass particles in the composition etch through the antireflection coating layer upon firing, helping to build contact between the metal and the n+ type silicon. On the other hand, the glass must not be so aggressive that it shunts the p-n junction after firing. Thus, the goal is to minimize contact resistance while keeping the p-n junction intact so as to achieve improved efficiency. Known compositions have high contact resistance due to the insulating effect of the glass in the interface of the metallic layer and silicon wafer, as well as other disadvantages such as high recombination in the contact area. Further, glass frits are known to have wide melting temperature ranges, making their behavior strongly dependent on the processing parameters.
Accordingly, there is a need for new electroconductive paste compositions with improved properties, such as flexible reactivity and thermal behavior.